The present invention relates to mechanical seismic restraint means which are used in nuclear safety systems. In particular, the seismic restraint means according to this invention is usable with generally cylindrical out-of-core nuclear radiation detectors which must be mechanically supported in a vertical position about a nuclear reactor vessel. This invention restrains the motion of a radiation detector during a seismic event and substantially absorbs vibration forces induced thereby which would otherwise be transmitted to the internal mechanism of the detector.
The typical out-of-core radiation detector utilized in nuclear safety systems is an ion chamber of substantial length, for example about 6 to 12 feet long. The ion chamber typically utilizes concentric cylindrical electrodes which are maintained a fixed distance apart, with opposed electrical potentials for attracting respectively the oppositely charged particles which are generated in the ion chamber by neutrons from the reactor core. Such ion chambers are thus used to monitor reactor activity and to indicate the operational status of the reactor. The ion chamber is typically mounted in an elongated tubular thimble which is typically open ended at the top and may be closed or open ended at the bottom. A plurality of such thimbles are spaced around the reactor vessel in a predetermined array to permit sampling of the neutron flux level in the vicinity of the reactor vessel. Seismic activity can result in the ion chamber striking the thimble wall generating significant forces which cause electrical noise to be present in the output signal from such ion chambers. This electrical noise is thought to be a result of the vibratory motion of the electrodes relative to each other in the ion chamber.
The tubular thimbles within which the ion chamber radiation detectors are typically mounted can have a variable inside diameter, as is typical for commercially available piping which is used in forming the reactor thimbles. A typical 6 inch nominal diameter schedule 80 piping typically will have an inside diameter which ranges from about 5.931 inch to 5.622 inch, and 6 inch nominal diameter schedule 40 pipe has a resultant inside diameter range of from about 6.197 inch to 5.964 inch. With such inside diameter range variations of up to about 0.2 inch inside diameter, it is difficult to provide a detector support means which provides a uniform fit and support of the detector assembly for the wide range of thimble inside diameters. It has thus been necessary to provide a rather loose fit in current detector support assemblies relative to the thimble ID. This leads to high levels of acceleration and deceleration of the detector assembly during seismic activity and results in undesirable electrical signals generated by motion of the internal detector parts. The presently utilized detector support assemblies are also rigid assemblies and there is thus no damping of impact shocks experienced by the assembly during seismic activity.
Additionally, the qualification of neutron detectors for use in the thimble requires that the detectors be capable of operating before, during and after a seismic event. Seismic event test sequences have been developed to conservatively simulate the seismic conditions predicted for a reactor. The capability of the neutron detector to survive the seismic test depends upon the energy absorption characteristics of the detector structure and the amount of energy transmitted to that structure. Internal damage to the detector mechanism and/or excessive and noisy signal outputs can result if either the detector or its support structure are unable to dissipate the seismic energy.
An example of a seismic restraint device is disclosed in U.S. patent application Ser. No. 382,437 filed May 26, 1982 which is assigned to the assignee of the present invention and which is incorporated herein by reference. The seismic restraint device utilizes spring bias means to mount a radiation detector in a tubular thimble.
It is therefore, an object of this invention to provide a seismic restraint means capable of absorbing and dissipating a significant fraction of the seismic energy that the detector can receive during a seismic event, either actual or simulated and can reduce electrical noise.
It is another object of the present invention to provide a seismic restraint means which maintains electrical isolation between the detector and the inside wall of the thimble.
It is also an object of this invention to provide a seismic restraint means which provides support for a detector assembly within a tubular thimble where the range of thimble inside diameters has significant variation.